Electrical insulation for magnetic bodies



COATING OR mane sip Patented Jan. 9, 1934 ames REFERENCE 'UNITED 'sT'A'r's PATENT OFFICE York, N. Y., a corporation of New York 8 Claims.

This invention relates to electrical insulation and particularly to theelectrical insulation of finely divided magnetic material in magneticbodies, such as magnetic cores for loading coils and the like.

The object of the invention is to improve the magnetic, electrical andmechanical characteristics of magnetic bodies of the type employingmagnetic material in finely divided form.

Magnetic cores, made from finely divided magnetic material, so-calledmagnetic dust cores, are extensively used for the loading coils whichare inserted in telephone lines to improve their transmissioncharacteristics. Such cores are usually constructed by subjectingmagnetic dust particles and an insulating and binding material betweenthe magnetic particles to extremely high pressures in the neighborhoodof 200,000 lbs. per square inch, so as to form the component materialsinto a substantially homogeneous mass in the desired core form, and togive sumcient mechanical strength to the completed core. The insulation,between the magnetic particles is provided, in

general, for the purpose of reducing eddy current losses in the core.

In the case especially where the magnetic material in a coremade in theabove described manner is a magnetic alloy including nickel and iron, ithas hitherto been found desirable to subject the core to a subsequentheat treatment at a high temperature to remove the strains introduced inthe magnetic material by the pressing operation, which tend to impairthe magnetic properties of the magnetic alloy in the core. It isdesirable, therefore, that the insulating material used in the core besuch as will'bind the magnetic materials firmly together, and such aswill withstand without deterioration the highpressures and temperaturesto which it will be subjected during the construction of the core. It isdesirable also that the insulating material in the core be such as willnot deteriorate under the conditions to which the loading coils aresubjected in service.

Inaccordance with the invention it has been A typical in accordance withthe ---s found that magnetic cores having extremely invention would havethe following weights of in- 100 2 good magnetic, electrical andmechanical characfsulating material per 100 grams'of magnetic dustteristics may be produced by use of insulating material: f

and

1.. gnesiaand s ap Other objects and the'advantages '6 invention willbecome apparent from the following de-'-- tailed description thereof.

In practicing the present invention, the magnetic material is preferablyprepa d from a brittle alloy containing nickel and iron, with or bindingmaterial comprising colldidallj la y,

Drawing. Application May 24, 1933 j Serial No. 672,643 I without theaddition of other constituents, and.

commonly known as rmalloy. The alloy may be prepared in th emannef'hescribed in Patent No. 1,669,649 issued May 15, 1928 to C. P.Beathand H. M. E: Heinicke, wherein approxi:

mately 81 parts oi! nickel and 19 parts of iron and other magneticmaterials are melted together in an oxidizing atmosphere and theresuiting alloy poured into a mold. The alloy thus,

broken into short pieces and the pieces reduced to more finely dividedform in any suitable manner, for example, by crushing in a rock crusher,hammer mill or other suitable apparatus, after which the crushedmaterial is reduced to dust in an attrition or ball mill. 'The resultingdust is ,tiien sifted and the portion passingthrough a 120 mesh sieve issuitable for use as core material.

That portion which will not pass through the 120 mesh sieve is returnedto the ball or attrition mill for further rolling or grinding, thisprocess being repeated until a suilicient quantity of the very fine dustis obtained. Prior to the addition of the insulating material, thefinely dividedparticles of the alloy so obtained are annealedin a closedcontainer at a temperature of aproximately 885 degrees C. The annealeddust is removed from the container in the form of a cake which isreduced to ness by crushing and grinding or rolling, and sifting througha 120 mesh sieve. The annealed dust a powderof suitable finev .is theninsulated with the insulating material '0! the invention in the mannerdescribed below.

A typical insulation in accordance with the insulating materialof.theinvention in the manner described below. E

Grams of magnesia. 2.11

Coll clay 0.77. ate 0.64

not

the dust, the kind of magnetic material used therein and the final corecharacteristics desired. Experiments indicate that for the best resultsin the production of cores for loading coils for low 6 frequencytelephone circuits, the amounts of the component insulating. materialswould be within the following limits: C l 0.1 to 1.1 grams, sodlsilicate, 0.1 to 1.3 grams and milk of magnes a, 6.3 grams per. 100grams 0 lil ma netic dust.

IE applying the insulation, the colloidal clay is first preferably mixedwith magnetic dust either dry or in a water suspension. The milk ofmagnesia is added to the mixture in a water suspension formed bydiluting the commercial product which comprises 32 to 40 grains of manesium hydroxide, Mg (OH) 2', per fluid ounce. A 50 per cent solution ofsodium silicate preferably having a silicate to soda ratio of about 1.6al- 'though other ratios have been found to give good results also, isadded to the mixturejn a dilute water solution. The additional water isprovided to insure complete mixing. Usually about 20 cc of liquid per100 grams of dust is sufficient. The

26 mixture is then evaporated to a condition of complete dryness'withconstant stirring to prevent caking and to insure a thorough coating ofthe individual dust particles. Toobtaln complete drying of the ma ticmateriaL'it may be 80 desirable to apply heat thereto at a temperatureof 120 C. or higher for a short time.

' The insulation may be applied all at one time or in stages, so as toobtain several layers of the sameinsulating materials on the magneticpar- 36 ticles, in the manner disclosed, for example, in

the patent to Lathrop 1,857,201, issued May 10, 1932. Three stages ofinsulation have been found to give very good results.

A variation of the multiple-coat method of insulatingdisclosed'in'tifehathioppatentfi'as been found to be particularlysuitable with the colloidal clay-milk of magnesiasodium silicateinsulation. According to this variation, one half .of the dilute watersuspension and the remaining 0.21 gram of sodium silicate in dilutewater solution are added to the mixture and the whole evaporated todryness to form the third layer of insulation on the magnetic particles.1 8( The magnetic dust particles insulated in the above escr ed mannerare en mixe with 0.38 gram of colloidal clay, placed in a mould andcompressed in 0 core parts under a pressure of approximately 200,000pounds per square inch. The core parts are then transferred to anannealing furnace where they are annealed at a high temperaturepreferably in hydrogen or in an, inert atmosphere to relieve theinternal stresses set up in the material by the pressing 9 operation,thereby producing a core having low hysteresis loss. Where the-annealingheat treatment is carried out in air, the core parts are preferablysubjected to a temperature of approximately 500 C. for about minutes.When the 9. annealing heat treatment is carried out in hydrogen whichenables higher annealing temperatures to be used, the core parts arepreferably subjected to a temperature of approximately 650 C. for aboutminutes. The usual loading 10 coil toroidal windings are wound on thesingle core parts thus produced or on a plurality of said core partsstacked coaxially. The number of core parts used in a given core willdepend upon the existing electrical characteristics of the telephonecircuit with which the loading coils are to be associated.- I

A large number of core parts made by the above described methods and inwhich the magnetic material is an alloy containing approximately 11 81per cent nickel, 17 per cent iron and 2 per cent molybdenum were testedby well known methods and found to have permeabilities ranging from 120to 160 and sufliciently low hysteresis and eddy current losses as to besatisfactory for 11 use as cores in coils for loading voice frequencytelephone circuits.

The colloidaLglay -milk of magnesia-sodium total amount of colloidalclay and all or the milk s ilicate f ns ulation of *tiiifiventiosis'aisosma 4B of magnesia and sodium silicate is used for applying theinsulating coatings to the .mmetic dust particles by the multiple-coatmethod, and then the coated magnetic particles are mixed with theremaining half of the colloidal clay andthe 60 mixture compressed toform the core part. For

example, where the insulating coatings are applied in three stages andthe proportions of the insulating material are as given above,the-procedure is as'follows: r

56 With every 100 grams of magnetic .dust particles, 0.13 gram ',ofcolloidal clay, dry or in a water suspension, is first mixed; then 0.70gram of the milk of magnesia in dilute water suspension and 0.21 gram ofsodium silicate in di- 60 lutewater solution are added to the 'able foruse in magnetic dustcores for coils em- 12 ployed for transm ng lg erfrequency signal currents, for example, carrier frequency and radiofrequency signal currents. As the proportions and kinds of magneticmetals used in the. magnetic alloy dust, the fineness of the dust andthe pressures and the temperatures of the annealing heat treatments inthe process for producing such cores may be different from those used inthe case of cores for low frequency coils, different amounts andrelative proportions of the constituentmaterials inthe insulation thanthose specified above may be required for the best results.

It is to be understood that the invention is not limited to theparticular magnetic materials and The resu tin mixture is t n evaporatedto a *the-specific roportions of the magnetic and incon t lllfimk fi msulating above mentioned, which were i'ingfto'apply the first insulatingcoating to the magnetic dust particles. The coatedmagnetic particles arethen mixed with an additional 0.1: gram of colloidal clay, an additional0.70 gram of milkof magnesia in a dilute water. suspension and anadditional 0.21 gram of sodium silicate in a dilute water solution areadded thereto and the whole evaporated again to dryness with constantstirring to form the second layer of insulation on .the magnetic dustparticles. Then the resulting coated magnetic dust particles are mixedwith 0.13 gram of colloidal clay, the re-" maining 0.70 gram'of milkofmagnesiain a fargiven by way of example only, but that it is of ageneric nature applicable to all magnetic materials and to variousproportions of the constituent insulating materials. It is to be understood also that the method of applying the insulating material may bevaried somewhat from that described above without departing from thespirit of the invention which is limited only within the scope oftheappended claims.

What is claimedis: a

1. A magnetic body comprising finely divided particles of a magneticmaterial and insulation between the particles produced by mixing the 1 IOATlNG R PLASl C 1f v,

COMPOSiTiflNS,

. and iron and insulation between the alloy par ticles produced bymixing with the alloy particles colloidal clay, milk of magnesia andsodium silicate in water and evaporating the mixture to dryness, thewhole compressed into a substantially homogeneous solid. and thereafterheat treated at a high temperature to improve the magneticcharacteristics of the alloy.

3. A magnetic body comprising finely divided particles of a magneticalloy comprising approximately 81 per cent nickel, 17 per cent iron and2 per cent molybdenum, and insulation between the alloy particles formedby mixing with said alloy particles and water approximately 0.7! gram ofcolloidal clay, 2.11 grams of commercial milk of magnesia and 0.64 gramoi a 50 per cent solution of sodium silicate for each grams of alloyparticles, and evaporating the mixture to dryness, the whole compressedinto a substantially homogeneous solid and subsequently annealed in aninert gas at a temperature of approximately 650 C. for about sixtyminutes.

4. The process of making a magnetic body which consists in mixing finelydivided magnetic material with colloidal clay, milk of magnesia andsodium silicate in a volatile liquid, evaporating the mixture to drynessto form insulating coatings on the magnetic particles, subjecting a massof the insulating particles to high pressure to form it into asubstantially homogeneous solid in the desired shape, and then heattreating the solid to improve the magnetic properties of the magneticmaterial therein.

5. The process 0! making a magnetic body which consists in first mixingfinely divided magnetic material with colloidal clay, mixing theresultant mixture with milk or magnesia andsodium silicate in a volatileliquid, evaporating the whole to dryness while stirring to forminsulating coatings on the magnetic particles, subiecting a mass of theinsulated particles to high pressure in a mold to produce asubstantially homogeneous solid in the desired form, and heat treatingthe solid to improve the magnetic properties of the magnetic materialtherein.

6. The process of making a magnetic body having high permeability andlow hysteresis and eddy current losses whichconsists in mixing with each100 grams '0! finely divided particles of a magnetic alloy comprisingapproximately 81 per cent nickel and 19 percent other m netic material,principally iron, approximately 0.38 gram oi colloidal clay, 2.11 gramscommercial milk of i magnesia and 0.64 gram oi a 50 per cent solu- CROSSmack tion or sodium silicate having a silicate to soda ratio 0!approximately 1.6, evaporating the mixture to, dryness while stirring toproduce insulating coatings on the individual magnetic particles, mixingthe coated particles with approximately 0.38 gram of colloidal clay foreach 100 grams of alloy, compressing the resulting mixture to form thecomponent materials into a substantially homogeneous solid andsubsequently heat treating said solid in an inert gas at a temperatureof approximately 650 C. for about one hour to improve the magneticcharacteristics for the alloy material therein.

'1. The process of making a mechanically strong magnetic body havinghigh permeability and low hysteresis and eddy current losses whichconsists in mixing with each 100 grams of finely divided magnetic alloycomprising approximately 81 per cent nickel, 17 per cent iron and 2 percent molybdeuum, approximately 0.13 gram of colloidal clay, 0.70 gramsof commercial milk of magnesia and 0.21 gram of a 50 per cent watersolution of sodium silicate in a volatile liquid and evaporating themixture to dryness while stirring to form a first insulating coating onthe magnetic 1uparticles, repeating this insulating process to produceon the insulated magnetic particlesa second and a third insulatingcoating each comprising the same kind and amount of insulating materialsas in the first coating, then mixing the 1 insulated magnetic particleswith approximately 0.39 gram of colloidal clay, subjecting a mass of theresulting mixture in a mold to a high pressure in the neighborhood of200,000 pounds per square inch to form the component materials into asubstantially homogeneous solid in the desired form and subjecting saidsolid to an annealing heat treatment in an. inert gas at a temperatureof approximately 650 C. for about sixty minutes to improve the magneticcharacteristics u of the magnetic alloy material therein.

8. A-magnetic core comprising'finely divided particles of a magneticalloy comprising, approximately 01 per cent nickel and 19 per cent othermagnetic metals principally iron, and insulation m between the particlesiormed by mixing with said alloy particles and water from 0.1 to 1.1grams colloidal clay, from 0.1 to 1.3 grams of a 50 per cent solution ofsodium silicate having a silicate Examiner to soda ratio 0!approximately 1.6, and from 0.25 1'v to 6.3 grams or commercial milk ofmagnesia for each 100 grams of alloy particles, and evaporating. themixture to dryness, the whole com pressed into a substantiallyhomogeneous solid and subsequently annealed at a high temperature I toimprove the magnetic characteristics or the alloy material therein.

'WILLIAM C. HHS.

